If it is being trained in a way so as to address self defense realistically, yes, it is.
That is not to say that 'sport' striking and blocking is useless in self defense; strikes and blocks are strikes and blocks. Plenty of people have defended themselves with boxing over the years, which is also sport.
The only real issue comes in how it is presented. If a school teaches striking in a competition setting, then they are teaching striking as it applies to competition, not to deal specifically with the sort of situations that can arise outside of the ring, and should be up front about that.
If a school teaches striking as it applies to realistic self defense scenarios, then they should likewise, say so. Even though it usually (though not always) comes across as a bit obnoxious, when a school makes a big deal out of not being sport, they're doing a service to all; those looking to become successful in competition will know right off the bat not to waist their time at that school.
Unfortunately, the SD only schools will proclaim that they don't do sport as loudly as they can from the highest rooftop, while sport only schools generally don't make similar proclamations about being 'sport only', as subconsciously, people still expect a martial art to teach self defense on some level, regardless of whether it is a reasonable expectation.
Daniel